Exploring the Combustion Modes of A Dual-Fuel Compression Ignition Engine

Abstract

Compression-ignition (CI) engines, also known as “diesel” engines, can produce higher thermal efficiency (TE) than spark-ignition (SI) engines, which allows them to emit less carbon dioxide (CO2) per unit of energy generated. Unfortunately, in practice the TE of CI engines is limited by the need to maintain sufficiently low emissions of nitrogen oxides (NOx) and soot. This problem can be mitigated by operating CI engines in dual-fuel modes with port fuel injection (PFI) of gasoline supplementing the direct injection (DI) of diesel fuel. Several strategies for doing this have been introduced in recent years, but these operating modes are usually studied individually at discrete conditions. This thesis introduces a classification system for dual-fuel CI modes that links together several previously studied modes across a continuous two-dimensional diagram. The combustion modes covered by this system include the standard modes of conventional diesel combustion (CDC) and conventional dual-fuel (CDF); the well-explored advanced combustion modes of HCCI, RCCI, PCCI, and PPCI; and a relatively unexplored combustion mode that is herein titled “Piston-split Dual-Fuel Combustion” or PDFC. The results show that dual-fuel CI engines can simultaneously increase TE and lower NOx and/or soot emissions at high loads through the use of Partial HCCI (PHCCI), despite an increase in emissions of carbon monoxide (CO) and unburnt hydrocarbons (UHC). At low loads, PHCCI is not possible, but either PDFC or RCCI can be used to further improve NOx and/or soot emissions, albeit at slightly lower TE and still higher emissions of CO and UHC. This multi-mode strategy of PHCCI at high loads and PDFC or RCCI at low loads is particularly useful when low engine-out NOx emissions are required.